1. Technical Field of the Invention
The present invention relates to synchronous rectifier boost converters, and more particularly to such devices in which the conventional boost diode is replaced by a bidirectional semiconductor switches are used
2. Relevant Art
Power factor correction (PFC) is required by international standards (EN61000-3-2) to reduce harmonic emissions in AC powered systems. The most common conventional solution employs an input rectifier bridge, followed by a boost switching converter, controlled by a voltage and a current loop. FIG. 1 shows a typical PFC rectifier stage, generally designated 10, including an input circuit 12, a diode bridge rectifier 14 feeding a MOSFET 16 through a boost choke 18, and a boost diode 20 which provides the output power through a capacitor 22. The load circuit, shown schematically as a resistor 24, is connected across capacitor 22.
Gate control for MOSFET 16, and PFC are provided by a suitable logic circuit 26. With this circuit, the voltage and current at the converter input 12 will be proportional at all times, generating the desired resistive behavior at the input of the system.
There are typically two problems that arise in boost topology configurations, namely high reverse recovery losses, and control of inrush current at startup. As to the first problem, when MOSFET 16 turns on during normal operation, the reverse recovery charge of boost diode 20 causes significant switching losses, seriously limiting the maximum switching frequency.
The second problem typically occurs at system startup, when the output capacitor 22 is discharged: the output capacitor is charged by the rectified AC line. The amplitude of the charging current is limited by the impedance of the input loop, resulting in a significant inrush current that can cause component failures.
In conventional topologies, there is no controllable switch in the path of the charging current by which the current path can be shut down if necessary. Conventional solutions for this problem take the form of negative temperature coefficient (NTC) or standard resistors with relays, SCR's, as illustrated on the input side of rectifier bridge in FIG. 1.
Specific conventional implementations of PFC in boost converters may be found in the following U.S. patents: U.S. Pat. No. 6,285,170 B1 to Matsumoto et al. for SWITCHING POWER SUPPLY; U.S. Pat. No. 5,420,780 to Bernstein et al. for APPARATUS FOR LIMITING INRUSH CURRENT; U.S. Pat. No. 5,994,882 to Ma for SYNCHRONOUS RECTIFIER FOR BOOST CONVERTERS. In all of these, however, the solutions address only one of the two main issues (inrush and reverse recovery losses) and they all use additional components which can not conveniently be fabricated as part of the converter IC. Clearly, a need exists for a better solution which addresses both problems, and also allows convenient integration.